The PV inverter is the key component of PV power systems. This equipment is required to convert the DC power generated by PV panels into AC power systems (DC-to-AC). It can be connected or not to grid-synchronized AC systems. Different converter circuits for single-phase and three-phase systems are often used in PV inverter applications. The existing PV power systems can be with transformer (galvanic isolation) or without transformer (transformer-less). The PV inverter must have low harmonic content in the voltage and current, i.e. provide voltage and current with a low total harmonic distortion, THDi and THDv, respectively, sent to the AC output power system, and controls the power factor to be close to unity.
The voltage source inverter (VSI) is often used for this purpose, especially at higher power levels. Also, multi-level VSI present lower voltage stress over the active switches, and are hence more suitable for high power levels. The greater the number of voltage levels, the lower is the voltage stress across the switches, and the switching losses are consequently reduced.
The developing trend of the PV inverter is high efficiency, high power density and lower cost. However, achieving high efficiency often results in lower power density, high cost and high weight/volume.
Most of the three-phase inverters found in the market today are based on the three-level neutral-point-clamping (NPC) inverter (also known as I-type) or the three-level active-NPC inverter (also known as T-type), as described in references 1 and 2.
The PV inverter systems are often using the multilevel topologies. Such topologies are described in reference 3.
When high power is being processed, semiconductors such as insulated gate bipolar transistors (IGBTs), Metal-Oxide-Semiconductor Field-Effect Transistors (MOSFETs), gate turn-off thyristors (GTOs), MOS-controlled thyristors (MCT), bipolar junction transistors (BJTs), junction gate field-effect transistors (JFETs), diodes, and others have been the chosen solution for the active switches in the applications found in the industry. However using those devices has been related with many issues that are limiting the efficiency and/or power density, such as current sharing between paralleled devices and reduction of the switching frequency due to the increased commutation losses which increases the weight and the size of the converters.
Another drawback of the existing solutions is the electromagnetic interference (EMI) levels which are too high and require output filter with several stages in order to reduce both common mode (CM) and differential mode (DM) noise, reducing the performance and increasing the volume/cost of the unit. The power factor (PF) is lower and the total harmonic distortion (THD) is higher. Also, the leakage current from PV array to ground is high (more than 300 milliamps (mA)).
The concept of the three-state switching cell is described in reference 4, and an application of this three-state switching cell with the object to increase the current capability of the three-level inverters is described in reference 5.
With this topological circuit it is possible to improve the performance of energy conversion from the PV array input to well-regulated AC output, including the maximum power point tracking (MPPT) functionality. Those topologies are for high current application and high power, with high efficiency compared to all classical three-level NPC inverters with silicon carbide switches. Furthermore, this topology has no limitation and can be used in other applications, such as an uninterruptible power supply (UPS), high-voltage direct current (HVDC) and AC motor drivers.
In PV systems this topological circuit can be applied as inverter to grid-connected or islanded applications with isolation transformer or transformer-less.
Achieving high efficiency in the topological circuits existing in the market today is possible with silicon carbide semiconductors, magnetic components and an amount of heavy/thick copper wires. This is however decreasing the power density and increasing the cost.
If the converter is going to be optimized for high power density, the efficiency will unavoidably be lower and the cost increases.
The choice for optimization in the existing solutions today is either efficiency or high power density, but never both.
With the circuits according to the present disclosure it is however possible to achieve both high efficiency and high power density at a low cost.